The present invention relates generally to electrophotography, more particularly, to a non-contact, single-component developing system and single-component toner that facilitates efficient development of an electrostatic image and consistent high quality image output.
Electrophotographic imaging process (or xerography) is a well-known method of copying or otherwise printing documents. In general, electrophotographic imaging uses a charge-retentive, photosensitive surface (known as a photoreceptor) that is initially charged uniformly. The photoreceptor is then exposed to a light image representation of a desired image that discharges specific areas of the photoreceptor surface creating a latent image. Toner powder is applied by using a developing system, which carries the toner from a toner container to the latent image, forming a developed image. This developed image is then transferred from the photoreceptor to a substrate (e.g. paper, transparency, and the like).
A color electrophotographic imaging process is typically achieved by repeating the same process described above for each color or tone of toner desired and storing each developed image to an accumulator until all desired colors or tones are achieved and then transferred to a substrate (e.g. paper, transparency, and the like).
There are several developing systems known in the art that carry the toner to the developing region and develop the latent image. One process is known as a xe2x80x9cnon-contactxe2x80x9d or xe2x80x9cjumpxe2x80x9d developing system. In operation, a thin layer of toner is adhered to a toner support member in spaced relation with respect to the latent image-bearing surface of the photoreceptor. When the toner is carried to the developing region between the toner support member and the photoreceptor, a bias voltage associated with the latent image areas of the photoreceptor tends to exert electrostatic forces that direct the toner particles towards the latent image areas on the surface of the photoreceptor. The electrostatic forces are often of insufficient magnitude to overcome the adhesion forces holding the toner particles in the thin layer on the toner support member. One solution is to apply high AC voltage to the developing region. The AC voltage agitates the toner particles to free them from the toner support member, enabling the toner particles to xe2x80x9cjumpxe2x80x9d the gap between the toner support member and the photoreceptor. The toner particles that jump the gap adhere to the latent image areas on the surface of the photoreceptor to form a developed image. For color or xe2x80x9ctone-on-tonexe2x80x9d developing, this process is repeated and the developed images containing individual colors are transferred to and stored on an accumulator until all desired colors or tones are achieved and than transferred to a substrate (e.g. paper, transparency, and the like). Although this process will produce color and tone-on-tone images with sufficient efficiency, the addition of an accumulator increases the complexity and cost of the electrophotographic imaging system.
Although previous efforts have been made to produce a non-contact developing system for multi-color imaging utilizing a single component toner and accumulation of the image on a single photoconductor (i.e., no accumulator), none of these efforts appear to have resulted in a system that effectively develops color toner particles to a photoreceptor with sufficient efficiency.
Also, previous efforts have been made to produce a non-contact developing system for monochrome imaging utilizing a single component toner and using DC bias only. None of these efforts appear to have resulted in a system that effectively develops toner particles to a photoreceptor with sufficient efficiency.
The present invention is directed to a non-contact, single-component developing system for electrophotographic machines that effectively reduces the impact of toner adhesion forces on the development process and facilitates toner jump while eliminating the need for AC voltages and, thus, an accumulator or some other intermediate transfer member. In a particularly innovative aspect, the developing system of the present invention utilizes a single-component toner that tends to reduce adhesion forces that tend to adhere toner particles to a toner support member. More particularly, the toner in accordance with the present invention includes large and small extraparticulate particles having concentrations by weight that preferably optimize surface coverage of the toner particles by the extraparticulate particles. In referring to surface coverage by area (surface coverage, surface coverage area), the total area of toner surface=xcfx80DT2 and the projected area of silica=Dsi2, as shown in FIG. 14. The extraparticulate particles of the present invention are preferably comprised of silica particles but may be comprised of an extraparticulate with similar physical characteristics to silica including material such as titanium dioxide, polymer microspheres, polymer beads, cerium oxide, zinc stearate, alumnina, and the like. In a preferred embodiment, surface coverage of toner particles by large extraparticulate particles is in a range of about 5 to 50 percent and surface coverage of toner particles by small extraparticulate particles is in a range of about 50 to 150 percent.
A toner may be prepared with the required calculated surface area coverage of extraparticulate particles by incorporation of a specific weight percent of each of the large and small extraparticulate particles by taking into account the mean diameter of the toner, the specific gravity of the toner and mean diameters and densities of each of the large and small extraparticulate particles. For example, for a 12 xcexc mean diameter toner with specific gravity of 1.1 g/cm3 combined with large extraparticulate particles having a mean diameter of 40 nm and a specific gravity of 2.2 g/cm3 and small extraparticulate having a mean diameter of 10 nm and specific gravity of 2.2 g/cm3, the surface area coverage of the large extraparticulate of 5 to 50 percent corresponds to a concentration by weight of 0.16 percent to 1.6 percent and the surface area coverage of the small extraparticulate of 50 to 150 percent corresponds to a concentration by weight of 0.45 to 1.35 percent.
In a further innovative aspect, the toner in accordance with the present invention has a development efficiency in a range of about 80 to 99 percent over a wide range of bias voltages.
In a preferred embodiment, a development system of the present invention preferably comprises a toner support member and a photoreceptor positioned in spaced relation. In operation, the photoreceptor is initially charged uniformly and then exposed to a light image representative of a desired image that discharges specific areas of the image bearing surface of the photoreceptor. Toner, which is carried to the developing region by the toner support member, is caused to jump the gap between the toner support member and the photoreceptor to the latent image, forming a developed image. Significantly, the electrostatic forces resulting from the DC bias voltage are sufficient to overcome toner adhesion forces without the use of AC voltages or some other means of freeing the toner free from the toner support member. This advantageously enables the development of color or xe2x80x9ctone-on-tonexe2x80x9d images without the need for an accumulator or some other intermediate transfer member.
Other innovative aspects of the invention include the preceding aspects individually or in combination.